It is well known to provide supplemental irrigation systems to assure that crops and other vegetation are provided with adequate water. This is especially important during times of little or irregular rainfall. It is important, however, to use the precious water resource efficiently and not to irrigate when it is unnecessary. As such, it is known to test soil conditions to determine moisture content. An approximate indication of soil moisture content can be readily obtained by simply observing the appearance and the consistency of the soil. If a more accurate, quantitative measurement is required, a common method of determining moisture content of soil is to obtain a soil sample and weigh it before and after oven drying. The sample is heated and dried to eliminate all moisture and the weight difference before and after represents the moisture in the soil. This method is slow and time consuming and is not very suitable for determining changing moisture content.
It is also known to determine the moisture content of soil by providing moisture sensing probes embedded in the soil. For example, U.S. Pat. No. 4,445,788 to Twersky et al shows a device for determining the moisture content, temperature and root development of soil. The device has helical threads at a lower end and several sensing stations along an outer tapered surface. The sensing stations include a camera and liquid crystals to transmit images to a viewing station above the surface through a series of fiber optic wires. A drawback of Twersky et al, in addition to its complexity, is that by driving the device down into the earth with the helical threads, the soil is compressed, which alters the amount of absorbed moisture in the soil and thus gives an inaccurate indication of soil moisture content.
Another type of moisture probe is a capacitance test probe such as shown by A. M. Thomas, "In Situ Measurement of Moisture in Soil and Similar Substances by Fringe Capacitance", J. Sci. Instrum., Vol. 43, 1966, pp. 21-27. The probe is square in cross section and includes two wedge shaped electrodes positioned on a pointed end of the probe. The probe is pushed down into the soil to measure the moisture content. However, pushing the probe down into the soil disturbs the soil and, as discussed above, impairs the accuracy of the measurement.
Other capacitance moisture probes have been proposed, such as Kuraz, "Testing of a Field Dielectric Soil Moisture Meter," Geotechnical Testing Journal, GTJODJ, Vol. 4, No. 3, September 1981, pp. 111-116, Wobschall, "A Frequency Shift Dielectric Soil Moisture Sensor," IEEE Transactions on Geoscience Electronics, Vol. GE-16, No. 2, April 1978, pp. 112-118 and Kuraz et al, "Resonance-Capacitance Soil Moisture Meter," Soil Science, Vol. 110, No. 4, 1970, pp. 278-279. These probes, however, are not designed to provide uniform contact with undisturbed soil.
Capacitance probes have also been used to measure the moisture content of materials other than soil. For example, U.S. Pat. No. 4,044,607 to Deal and U.S. Pat. No. 4,399,404 to Resh disclose capacitance moisture sensing devices used for grain storage units. These types of probes however are unsuitable for measuring the moisture content of soil because of the intrinsic differences between ground soil and stored grain. Grain is fairly light in weight and does not compact like soil. Therefore, a probe may be pushed and rocked down into the grain unit and the grain will flow in, around, and with the probe to maintain contact with the probe on all sides. However, driving a probe down into soil will compact the soil and alter the amount of absorbed moisture therein. Also, when the soil is compacted, it generally retains the compacted shape. Therefore, if the probe is rocked into place, the probe will not be in proper uniform contact with the soil to obtain an accurate measurement.
Accordingly, it is an object of the present invention to provide a method of determining the moisture content of soil which avoids the aforementioned drawbacks of the prior art.
It is a more particular object of the present invention to provide a method of determining the moisture content of soil which provides a more reliable and more accurate measurement and which is simple and efficient.
It is also an object of the present invention to provide a durable capacitance moisture sensing probe having the electrodes along the tapered outer surface portion to provide easier and reliable contact with the soil.